Respond to API council feedback.

Move aggressive allocation to @SystemApi, which means we can hide
the "flags" API variants.

Remove UUID APIs, since we should use existing Serializable APIs.

Relax permission checks to allow apps to ask for their own stats.

Improve docs.

Test: cts-tradefed run commandAndExit cts-dev -m CtsAppSecurityHostTestCases -t android.appsecurity.cts.StorageHostTest
Bug: 37534687, 37534941, 37718184, 37738770
Change-Id: I6a763fb3ab3169c8d3329765bb31e1ee08d9ced7